This limit is hard-coded in the stylesheet file $InstallationDirectory\SystemFiles\FrontEnd\StyleSheets\Utility\Outline.nb (you can view the plain text content if you open it with, say, Notepad). You can probably make your own stylesheet with any number of levels using Outline.nb as the starting point, but ... this involves more than adding a few lines to this file.
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kglrOct 11 '12 at 17:24

1

I would copy-paste the Outline6 style in the stylesheet as a reference and then just change the name to Outline7, increasing the left CellMargins by 40 or whatever, changing the style according to taste, adding Outline7 to Outline6's StyleKeyMapping so that Tabbing the Outline6 brings you to Outline7, changing Outline7's StyleKeyMapping so that backspace brings you to Outline6. Also, add Outline7 to the DockedCells list in the Working style cell
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RojoOct 21 '12 at 16:13

1 Answer
1

In the following, I take Outline.nb file and insert a cell-group to add a 7th level and modify the remaining cells (1) to add the new level to the toolbar, (2) to modify the current level 6 behavior in response to Tab and Shift+Tab key events, (3) for all levels, reset the counter values to zero for all child levels including level 7.

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